This invention relates to improved materials usable as electrode active materials and to their preparation.
Lithium batteries are prepared from one or more lithium electrochemical cells containing electrochemically active (electroactive) materials. Such cells typically include an anode (negative electrode), a cathode (positive electrode), and an electrolyte interposed between spaced apart positive and negative electrodes. Batteries with anodes of metallic lithium and containing metal chalcogenide cathode active material are known. The electrolyte typically comprises a salt of lithium dissolved in one or more solvents, typically nonaqueous (aprotic) organic solvents. Other electrolytes are solid electrolytes typically called polymeric matrixes that contain an ionic conductive medium, typically a metallic powder or salt, in combination with a polymer that itself may be ionically conductive which is electrically insulating. By convention, during discharge of the cell, the negative electrode of the cell is defined as the anode. Cells having a metallic lithium anode and metal chalcogenide cathode are charged in an initial condition. During discharge, lithium ions from the metallic anode pass through the liquid electrolyte to the electrochemical active (electroactive) material of the cathode whereupon they release electrical energy to an external circuit.
It has recently been suggested to replace the lithium metal anode with an insertion anode, such as a lithium metal chalcogenide or lithium metal oxide. Carbon anodes, such as coke and graphite, are also insertion materials. Such negative electrodes are used with lithium-containing insertion cathodes, in order to form an electroactive couple in a cell. Such cells, in an initial condition, are not charged. In order to be used to deliver electrochemical energy, such cells must be charged in order to transfer lithium to the anode from the lithium-containing cathode. During discharge the lithium is transferred from the anode back to the cathode. During a subsequent recharge, the lithium is transferred back to the anode where it re-inserts. Upon subsequent charge and discharge, the lithium ions (Li+) are transported between the electrodes. Such rechargeable batteries, having no free metallic species are called rechargeable ion batteries or rocking chair batteries. See U.S. Pat. Nos. 5,418,090; 4,464,447; 4,194,062; and 5,130,211.
Preferred positive electrode active materials include LiCoO2, LiMn2O4, and LiNiO2. The cobalt compounds are relatively expensive and the nickel compounds are difficult to synthesize. A relatively economical positive electrode is LiMn2O4, for which methods of synthesis are known. The lithium cobalt oxide (LiCoO2), the lithium manganese oxide (LiMn2O4), and the lithium nickel oxide (LiNiO2) all have a common disadvantage in that the charge capacity of a cell comprising such cathodes suffers a significant loss in capacity. That is, the initial capacity available (amp hours/gram) from LiMn2O4, LiNiO2, and LiCoO2 is less than the theoretical capacity because significantly less than 1 atomic unit of lithium engages in the electrochemical reaction. Such an initial capacity value is significantly diminished during the first cycle operation and such capacity further diminishes on every successive cycle of operation. For LiNiO2 and LiCoO2 only about 0.5 atomic units of lithium is reversibly cycled during cell operation. Many attempts have been made to reduce capacity fading, for example, as described in U.S. Pat. No. 4,828,834 by Nagaura et al. However, the presently known and commonly used, alkali transition metal oxide compounds suffer from relatively low capacity. Therefore, there remains the difficulty of obtaining a lithium-containing electrode material having acceptable capacity without disadvantage of significant capacity loss when used in a cell.
The invention provides novel lithium-mixed metal materials which, upon electrochemical interaction, release lithium ions, and are capable of reversibly cycling lithium ions. The invention provides a rechargeable lithium battery which comprises an electrode formed from the novel lithium-mixed metal materials. Methods for making the novel lithium-mixed metal materials and methods for using such lithium-mixed metal materials in electrochemical cells are also provided. The lithium-mixed metal materials comprise lithium and at least one other metal besides lithium. Preferred materials are lithium-mixed metal phosphates which contain lithium and two other metals besides lithium. Accordingly, the invention provides a rechargeable lithium battery which comprises an electrolyte; a first electrode having a compatible active material; and a second electrode comprising the novel materials. In one aspect, the novel materials are lithium-mixed metal phosphates which preferably used as a positive electrode active material, reversibly cycle lithium ions with the compatible negative electrode active material. Desirably, the lithium-mixed metal phosphate is represented by the nominal general formula LiaMIbMIIc(PO4)d. Such compounds include Li1MIaMIIbPO4 and Li3MIaMIIb(PO4)3; therefore, in an initial condition 0xe2x89xa6axe2x89xa61 or 0xe2x89xa6axe2x89xa63, respectively. During cycling, x quantity of lithium is released where 0xe2x89xa6xxe2x89xa6a. In the general formula, the sum of b plus c is up to about 2. Specific examples are Li1MI1xe2x88x92yMIIyPO4 and Li3MI2xe2x88x92yMIIy(PO4)3.
In one aspect, MI and MII are the same. In a preferred aspect, MI and MII are different from one another. At least one of MI and MII is an element capable of an oxidation state higher than that initially present in the lithium-mixed metal phosphate compound. Correspondingly, at least one of MI and MII has more than one oxidation state in the phosphate compound, and more than one oxidation state above the ground state M0. The term oxidation state and valence state are used in the art interchangeably.
In another aspect, both MI and MII may have more than one oxidation state and both may be oxidizable from the state initially present in the phosphate compound. Desirably, MII is a metal or semi-metal having a +2 oxidation state, and is selected from Groups 2, 12 and 14 of the Periodic Table. Desirably, MII is selected from non-transition metals and semi-metals. In one embodiment, MII has only one oxidation state and is nonoxidizable from its oxidation state in the lithium-mixed metal compound. In another embodiment, MII has more than one oxidation state. Examples of semi-metals having more than one oxidation state are selenium and tellurium; other non-transition metals with more than one oxidation state are tin and lead. Preferably, MII is selected from Mg (magnesium), Ca (calcium), Zn (zinc), Sr (strontium), Pb (lead), Cd (cadmium), Sn (tin), Ba (barium), and Be (beryllium), and mixtures thereof. In another preferred aspect, MII is a metal having a +2 oxidation state and having more than one oxidation state, and is oxidizable from its oxidation state in lithium-mixed metal compound.
Desirably, MI is selected from Fe (iron), Co (cobalt), Ni (nickel), Mn (manganese), Cu (copper), V (vanadium), Sn (tin), Ti (titanium), Cr (chromium), and mixtures thereof. As can be seen, MI is preferably selected from the first row of transition metals and further includes tin, and MI preferably initially has a +2 oxidation state.
In a preferred aspect, the product LiMI1xe2x88x92yMIIyPO4 is an olivine structure and the product Li3MI2xe2x88x92yMIIy(PO4)3 is a rhombohedral or monoclinic Nasicon structure. In another aspect, the term xe2x80x9cnominal formulaxe2x80x9d refers to the fact that the relative proportion of atomic species may vary slightly on the order of 2 percent to 5 percent, or more typically, 1 percent to 3 percent. In still another aspect, any portion of P (phosphorous) may be substituted by Si (silicon), S (sulfur), and/or As (arsenic); and any portion of O (oxygen) may be substituted by halogen, preferably F (fluorine). These aspects are also disclosed in U.S. patent application Ser. Nos. 09/105,748 filed Jun. 26, 1998, and 09/274,371 filed Mar. 23, 1999; and in U.S. Pat. No. 5,871,866 issued Feb. 16, 1999, which is incorporated by reference in its entirety; each of the listed applications and patents are co-owned by the assignee of the present invention.
The metal phosphates are alternatively represented by the nominal general formulas such as Li1xe2x88x92xMI1xe2x88x92yMIIyPO4 (0xe2x89xa6xxe2x89xa61), and Li3xe2x88x92xMI2xe2x88x92yMIIy(PO4)3 signifying capability to release and reinsert lithium. The term xe2x80x9cgeneralxe2x80x9d refers to a family of compounds, with M, x and y representing variations therein. The expressions 2xe2x88x92y and 1xe2x88x92y each signify that the relative amount of MI and MII may vary. In addition, as stated above, MI may be a mixture of metals meeting the earlier stated criteria for MI. In addition, MII may be a mixture of metallic elements meeting the stated criteria for MII. Preferably, where MII is a mixture, it is a mixture of 2 metallic elements; and where MI is a mixture, it is a mixture of 2 metals. Preferably, each such metal and metallic element has a +2 oxidation state in the initial phosphate compound.
The active material of the counter electrode is any material compatible with the lithium-mixed metal phosphate of the invention. Where the lithium-mixed metal phosphate is used as a positive electrode active material, metallic lithium, lithium-containing material, or non-lithium-containing material may be used as the negative electrode active material. The negative electrode is desirably a nonmetallic insertion material. Desirably, the negative electrode comprises an active material from the group consisting of metal oxide, particularly transition metal oxide, metal chalcogenide, carbon, graphite, and mixtures thereof. It is preferred that the anode active material comprises a carbonaceous material such as graphite. The lithium-mixed metal phosphate of the invention may also be used as a negative electrode material.
In another embodiment, the present invention provides a method of preparing a compound of the nominal general formula LiaMIbMIIc(PO4)d where 0 less than axe2x89xa63; the sum of b plus c is greater than zero and up to about 2; and 0 less than dxe2x89xa63. Preferred compounds include Li3MIbMIIc(PO4)3 where b plus c is about 2; and LiMIbMIIcPO4 where b plus c is about 1. The method comprises providing starting materials in particle form. The starting (precursor) materials include a lithium-containing compound, one or more metal containing compounds, a compound capable of providing the phosphate (PO4)xe2x88x921 anion, and carbon. Preferably, the lithium-containing compound is in particle form, and an example is lithium salt. Preferably, the phosphate-containing anion compound is in particle form, and examples include metal phosphate salt and diammonium hydrogen phosphate (DAHP) and ammonium dihydrogen phosphate (ADHP). The lithium compound, one or more metal compounds, and phosphate compound are included in a proportion which provides the stated nominal general formula. The starting materials are mixed together with carbon, which is included in an amount sufficient to reduce the metal ion of one or more of the metal-containing starting materials without full reduction to an elemental metal state. Excess quantities of carbon and one or more other starting materials (i.e., 5 to 10% excess) may be used to enhance product quality. A small amount of carbon, remaining after the reaction, functions as a conductive constituent in the ultimate electrode formulation. This is an advantage since such remaining carbon is very intimately mixed with the product active material. Accordingly, large quantities of excess carbon, on the order of 100% excess carbon are useable in the process. The carbon present during compound formation is thought to be intimately dispersed throughout the precursor and product. This provides many advantages, including the enhanced conductivity of the product. The presence of carbon particles in the starting materials is also thought to provide nucleation sites for the production of the product crystals.
The starting materials are intimately mixed and then reacted together where the reaction is initiated by heat and is preferably conducted in a nonoxidizing, inert atmosphere, whereby the lithium, metal from the metal compound(s), and phosphate combine to form the LiaMIbMIIc(PO4)d product. Before reacting the compounds, the particles are intermingled to form an essentially homogeneous powder mixture of the precursors. In one aspect, the precursor powders are dry-mixed using a ball mill, such as zirconia media. Then the mixed powders are pressed into pellets. In another aspect, the precursor powders are mixed with a binder. The binder is selected so as to not inhibit reaction between particles of the powders. Therefore, preferred binders decompose or evaporate at a temperature less than the reaction temperature. Examples include mineral oils (i.e., glycerol, or C-18 hydrocarbon mineral oil) and polymers which decompose (carbonize) to form a carbon residue before the reaction starts, or which evaporate before the reaction starts. In still another aspect, intermingling is conducted by forming a wet mixture using a volatile solvent and then the intermingled particles are pressed together in pellet form to provide good grain-to-grain contact.
Although it is desired that the precursor compounds be present in a proportion which provides the stated general formula of the product, the lithium compound may be present in an excess amount on the order of 5 percent excess lithium compared to a stoichiometric mixture of the precursors. And the carbon may be present at up to 100% excess compared to the stoichiometric amount. The method of the invention may also be used to prepare other novel products, and to prepare known products. A number of lithium compounds are available as precursors, such as lithium acetate (LiOOCCH3), lithium hydroxide, lithium nitrate (LiNO3), lithium oxalate (Li2C2O4) lithium oxide (Li2O), lithium phosphate (Li3PO4), lithium dihydrogen phosphate (LiH2PO4), lithium vanadate (LiVO3), and lithium carbonate (Li2CO3). The lithium carbonate is preferred for the solid state reaction since it has a very high melting point and commonly reacts with the other precursors before melting. Lithium carbonate has a melting point over 600xc2x0 C. and it decomposes in the presence of the other precursors and/or effectively reacts with the other precursors before melting. In contrast, lithium hydroxide melts at about 400xc2x0 C. At some reaction temperatures preferred herein of over 450xc2x0 C. the lithium hydroxide will melt before any significant reaction with the other precursors occurs to an effective extent. This melting renders the reaction very difficult to control. In addition, anhydrous LiOH is highly hygroscopic and a significant quantity of water is released during the reaction. Such water needs to be removed from the oven and the resultant product may need to be dried. In one preferred aspect, the solid state reaction made possible by the present invention is much preferred since it is conducted at temperatures at which the lithium-containing compound reacts with the other reactants before melting. Therefore, lithium hydroxide is useable as a precursor in the method of the invention in combination with some precursors, particularly the phosphates. The method of the invention is able to be conducted as an economical carbothermal-based process with a wide variety of precursors and over a relatively broad temperature range.
The aforesaid precursor compounds (starting materials) are generally crystals, granules, and powders and are generally referred to as being in particle form. Although many types of phosphate salts are known, it is preferred to use diammonium hydrogen phosphate (NH4)2HPO4 (DAHP) or ammonium dihydrogen phosphate NH4H2PO4 (ADHP). Both ADHP and DAHP meet the preferred criteria that the precursors decompose in the presence of one another or react with one another before melting of such precursor. Exemplary metal compounds are Fe2O3, Fe3O4, V2O5, VO2, LiVO3, NH4VO3, Mg(OH)2, CaO, MgO, Ca(OH)2, MnO2, Mn2O3, Mn3(PO4)2, CuO, SnO, SnO2, TiO2, Ti2O3, Cr2O3, PbO2, PbO, Ba(OH)2, BaO, Cd(OH)2. In addition, some starting materials serve as both the source of metal ion and phosphate, such as FePO4, Fe3(PO4)2, Co3(PO4)2, Zn3(PO4)2, and Mg3(PO4)2. Still others contain both lithium ion and phosphate such as Li3PO4 and LiH2PO4. Other exemplary precursors are H3PO4 (phosphoric acid); and P2O5 (P4O10) phosphoric oxide; and HPO3 meta phosphoric acid, which is a decomposition product of P2O5. If it is desired to replace any of the oxygen with a halogen, such as fluorine, the starting materials further include a fluorine compound such as LiF. If it is desired to replace any of the phosphorous with silicon, then the starting materials further include silicon oxide (SiO2). Similarly, ammonium sulfate in the starting materials is useable to replace phosphorus with sulfur.
The starting materials are available from a number of sources. The following are typical. Vanadium pentoxide of the formula V2O5 is obtainable from any number of suppliers including Kerr McGee, Johnson Matthey, or Alpha Products of Davers, Mass. Vanadium pentoxide has a CAS number of 1314-62-1. Iron oxide Fe3O3 is a common and very inexpensive material available in powder form from the same suppliers. The other precursor materials mentioned above are also available from well known suppliers, such as those listed above.
The method of the invention may also be used to react starting materials in the presence of carbon to form a variety of other novel products, such as gamma-LiV2O5 and also to produce known products. Here, the carbon functions to reduce metal ion of a starting metal compound to provide a product containing such reduced metal ion. The method is particularly useful to also add lithium to the resultant product, which thus contains the metallic element ions, namely, the lithium ion and the other metal ion, thereby forming a mixed metal product. An example is the reaction of vanadium pentoxide (V2O5) with lithium carbonate in the presence of carbon to form gamma-LiV2O5. Here the starting metal ion V+5V+5 is reduced to V+4V+5 in the final product. A single phase gamma-LiV2O5 product is not known to have been directly and independently formed before.
As described earlier, it is desirable to conduct the reaction at a temperature where the lithium compound reacts before melting. The temperature should be about 400xc2x0 C. or greater, and desirably 450xc2x0 C. or greater, and preferably 500xc2x0 C. or greater, and generally will proceed at a faster rate at higher temperatures. The various reactions involve production of CO or CO2 as an effluent gas. The equilibrium at higher temperature favors CO formation. Some of the reactions are more desirably conducted at temperatures greater than 600xc2x0 C.; most desirably greater than 650xc2x0 C.; preferably 700xc2x0 C. or greater; more preferably 750xc2x0 C. or greater. Suitable ranges for many reactions are about 700 to 950xc2x0 C., or about 700 to 800xc2x0 C.
Generally, the higher temperature reactions produce CO effluent and the stoichiometry requires more carbon be used than the case where CO2 effluent is produced at lower temperature. This is because the reducing effect of the C to CO2 reaction is greater than the C to CO reaction. The C to CO2 reaction involves an increase in carbon oxidation state of +4 (from 0 to 4) and the C to CO reaction involves an increase in carbon oxidation state of +2 (from ground state zero to 2). Here, higher temperature generally refers to a range of about 650xc2x0 C. to about 1000xc2x0 C. and lower temperature refers to up to about 650xc2x0 C. Temperatures higher than 1200xc2x0 C. are not thought to be needed.
In one aspect, the method of the invention utilizes the reducing capabilities of carbon in a unique and controlled manner to produce desired products having structure and lithium content suitable for electrode active materials. The method of the invention makes it possible to produce products containing lithium, metal and oxygen in an economical and convenient process. The ability to lithiate precursors, and change the oxidation state of a metal without causing abstraction of oxygen from a precursor is heretofore unexpected. These advantages are at least in part achieved by the reductant, carbon, having an oxide whose free energy of formation becomes more negative as temperature increases. Such oxide of carbon is more stable at high temperature than at low temperature. This feature is used to produce products having one or more metal ions in a reduced oxidation state relative to the precursor metal ion oxidation state. The method utilizes an effective combination of quantity of carbon, time and temperature to produce new products and to produce known products in a new way.
Referring back to the discussion of temperature, at about 700xc2x0 C. both the carbon to carbon monoxide and the carbon to carbon dioxide reactions are occurring. At closer to 600xc2x0 C. the C to CO2 reaction is the dominant reaction. At closer to 800xc2x0 C. the C to CO reaction is dominant. Since the reducing effect of the C to CO2 reaction is greater, the result is that less carbon is needed per atomic unit of metal to be reduced. In the case of carbon to carbon monoxide, each atomic unit of carbon is oxidized from ground state zero to plus 2. Thus, for each atomic unit of metal ion (M) which is being reduced by one oxidation state, one half atomic unit of carbon is required. In the case of the carbon to carbon dioxide reaction, one quarter atomic unit of carbon is stoichiometrically required for each atomic unit of metal ion (M) which is reduced by one oxidation state, because carbon goes from ground state zero to a plus 4 oxidation state. These same relationships apply for each such metal ion being reduced and for each unit reduction in oxidation state desired.
It is preferred to heat the starting materials at a ramp rate of a fraction of a degree to 10xc2x0 C. per minute and preferably about 2xc2x0 C. per minute. Once the desired reaction temperature is attained, the reactants (starting materials) are held at the reaction temperature for several hours. The heating is preferably conducted under non-oxidizing or inert gas such as argon or vacuum. Advantageously, a reducing atmosphere is not required, although it may be used if desired. After reaction, the products are preferably cooled from the elevated temperature to ambient (room) temperature (i.e., 10xc2x0 C. to 40xc2x0 C.). Desirably, the cooling occurs at a rate similar to the earlier ramp rate, and preferably 2xc2x0 C./minute cooling. Such cooling rate has been found to be adequate to achieve the desired structure of the final product. It is also possible to quench the products at a cooling rate on the order of about 100xc2x0 C./minute. In some instances, such rapid cooling (quench) may be preferred.
The present invention resolves the capacity problem posed by widely used cathode active material. It has been found that the capacity and capacity retention of cells having the preferred active material of the invention are improved over conventional materials. Optimized cells containing lithium-mixed metal phosphates of the invention potentially have performance improved over commonly used lithium metal oxide compounds. Advantageously, the new method of making the novel lithium-mixed metal phosphate compounds of the invention is relatively economical and readily adaptable to commercial production.
Objects, features, and advantages of the invention include an electrochemical cell or battery based on lithium-mixed metal phosphates. Another object is to provide an electrode active material which combines the advantages of good discharge capacity and capacity retention. It is also an object of the present invention to provide electrodes which can be manufactured economically. Another object is to provide a method for forming electrode active material which lends itself to commercial scale production for preparation of large quantities.
These and other objects, features, and advantages will become apparent from the following description of the preferred embodiments, claims, and accompanying drawings.